1. Field of the Invention
This invention relates to an oscillator which has low noise and in particular low phase noise in the vicinity of the center frequency.
2. Description of the Prior Art
A conventional oscillator comprises a resonator constituted for example by an inductance coil and a capacitor, an amplifier, and a non-linear element. The amplifier excites the resonator by supplying it with energy. Part of the energy stored in the resonator is collected so as to be applied to the input of the amplifier. The power delivered by the output of the amplifier is higher than the sum of the power dissipated by Joule effect within the resonator and of the power applied to the input of the ampliier. The amplitude of the output voltage of the amplifier would therefore increase indefinitely if provision were not made in said loop for a nonlinear element which reduces the gain of the loop when the amplitude of said voltage increases. The nonlinear element of the oscillator is therefore essential to ensure stable oscillation. The nonlinear element is often constituted by the amplifier itself since it becomes nonlinear when it saturates.
The modelization of an oscillator of this type is described by:
Leeson in "Simple model of a feedback oscillator noise spectrum" Proc. IEEE Vol. 54, Feb. 1966; PA0 H. J. Siweris, B. Chiek in "Analysis of noise up conversion in microwave FET oscillators" IEEE Trans MTT, vol MTT-33 pp 233-242, Mar. 1985.
This modelization serves to show that the noise in the vicinity of the center frequency in this type of oscillator is essentially due to conversion of the low-frequency noise of the amplifier to high-frequency noise by a reversal effect caused by non-linearity of operation of the amplifier. Since the amplifier is usually a semiconductor device having relatively high noise at low frequencies, this type of oscillator has relatively high phase noise.
It is known to reduce the noise of an oscillator by making use of a resonator having a very high quality factor. This solution, however, is often complex and costly. Furthermore, it is not practicable in the case of a voltage-controlled oscillator since an oscillator of this type has a variable-capacitance diode which damps the resonator.
As disclosed by M. Mamodaly, M. Prigent, J. Obregon in "New configurations to reduce converted FM noise in FET oscillators", it is already known to separate the nonlinear element and the amplifier in order to reduce the noise of this type of oscillator. A nonlinear element as provided in a known device comprises two diodes connected in top-to-tail relation in parallel with the resonator in order to limit the voltage at the terminals of the resonator.
Another known device comprises a linear amplifier and a device for automatically controlling the gain of said amplifier, thereby permitting automatic adjustment of gain so as to provide exact compensation for losses in the other elements of the oscillator, especially the resonator, while maintaining the amplifier in a linear operating zone. These two known devices serve to provide an oscillator having lower noise than the conventional oscillator which makes use of the amplifier as an amplitude limiter. However, this reduction of noise is still insufficient for certain applications and inapplicable in some instances.